In recent years, utilizing a SNOM (Scanning-Near-Field-Optical-Microscope) realizing formation of an ultra resolving power image exceeding the limitation of an optical wavelength, researches are made to develop a high-density memory (SNOM-type memory) of 60 Gbit/in.sup.2 -1 Tbit/in.sup.2 passing the limitation of the conventional optical or magnetic recording performance.
To achieve the generation of the ultra resolving power image by near-field-optics, it is necessary to utilize an electromagnetic field called an evanescent field. In the SNOM-type memory, as a method of generating and detecting the evanescent field, an optical fiber probe and a hollow probe made of a dielectric substance are usually used. These probes have an end part at its tip having a length shorter than the wavelength of light that is incident thereon. The light leaves the probe from the end part as the evanescent field. It is known that the resolving power of the probe is almost equal to the length of the end part. The ultra resolving power image can be obtained by utilizing such a probe.
Accordingly, as the probe for use in the SNOM-type memory intended to obtain high-density recording, it is necessary to optically form an aperture so designed that the aperture has a diameter almost equal to a required resolving power. The intensity of the light leaving the tip of the probe is another factor determining a signal quality (S/N) and recording density. Thus, in the probe for use in the high-density memory, it is important to allow the incident light to reach the tip of the probe without the incident light leaking out from the probe.
Research has been hitherto made to prevent light from leaking out from the probe by forming a thin metal film on the outer surface of the probe. The use of a semiconductor laser is most favorable as the light source of the high-density memory. The wavelength of the semiconductor laser is 635 nm, 650 nm, 780 nm, and 830 nm which are more than 600 nm. Therefore, as the thin metal film covering the probe, a material having a high reflectivity for light with a wavelength more than 600 nm is desired. As the thin metal film having a high reflectivity for the light with the wavelength more than 600 nm, gold (Au) and copper (Cu) are described in Japanese Patent Application Laid-Open Publication No.8-94649. It is a well-known fact that Au and Cu indicate a high reflectivity for the light with the wavelength more than 600 nm as described in "New Edition of Table of Physical constants, table 7.1.2.3. P.172" or the like.
However, in an experiment in which the Au film having a high reflectivity is formed and light emitted by an He--Ne laser (632.8 nm) was incident on one end of the probe, the incident light was unable to reach the tip of the probe without the incident light leaking out from the probe. Thus, the intensity of the light at the tip of the probe is remarkably reduced, and the signal quality (S/N) is inferior. Accordingly, the experiment has revealed a problem that the probe is incapable of accomplishing high-density recording.